The present invention relates to a method and apparatus for molding a magnetic tape cassette, and particularly to a method and apparatus for molding the casing body of a magnetic tape cassette in a plurality of colors.
In a magnetic tape cassette, a pair of hubs on which a magnetic tape is wound are rotatably supported in a casing body composed of upper and lower half portions manufactured by the injection molding of a plastic material. The upper and lower half portions are provided with transparent or semitransparent windows through which the state of winding of the magnetic tape can be observed. If the entire casing body is made of a light-transmissible material, the light transmissibility of the windows is made higher than that of the other parts of the two half portions to make the interior of the casing body more readily visible through the windows. The windows are formed in a desired shape so as to obtain a good overall appearance.
In one type of magnetic tape cassette, the windows are manufactured separately from the casing body of the cassette and are attached to the body by an adhesive, ultrasonic fuse-bonding, or the like. That is, the windows must be manufactured separately from the casing body and then attached thereto. If the casing body is made of a light-transmissible material, the attaching parts of the windows are unavoidably clearly visible, disturbing the appearance of the cassette. For these reasons, a magnetic tape cassette molding method in which the casing body of a magnetic tape cassette is directly molded from plastics in two colors has been frequently employed.
However, since the retention force between the joined surfaces of the two types of plastics constituting the casing body of such a magnetic tape cassette depends on the heat and pressure which are applied to the molten plastic materials when they are subjected to injection molding to form the casing body, the strength of the casing body at the joined surfaces is often insufficient. Particularly, if the thickness of all solid parts of the casing body is small or the plastics are of such kinds as not to be soluble in each other, the problem is very serious with respect to the quality and service life of the cassette.
In order to solve this problem, a construction in which the joined surfaces are stepwise shaped to increase the contact areas thereof has been proposed. However, this approach is not adequate to fully solve the above problem.
In another attempt to solve the problem, a magnetic tape cassette has been proposed in which one of the joined surfaces of the window and the other part of a casing body is provided with a projection at the middle of the thickness of the casing body, and the other of the surfaces is provided with a recess for receiving the projection. Such a cassette is disclosed in Japanese Unexamined Published Utility Model Application No. 60083/85. The projection is molded with the use of a slide core. The slide core, which is slidable in the direction of thickness of the casing body of the cassette, is provided with a recess which extends in the direction of the length or width of the casing body to mold the projection. After molding, the recess of the slide core and the projection made of the injected plastic are separated from each other by sliding the slide core. However, a large amount of strain is created when the slide core is moved. For that reason, the size of the projection and recess of the joined surfaces cannot be made sufficiently large. That is, if the sizes of these members are increased to increase the holding force therebetween, the projection would be overly strained or deformed due to the movement of the slide core.